最近一直在学习XML的Xpath解析方式，据说是一个很简单的遍历XML文件的工具，类似于SQL和Oracle的关系，但是找了很多都没有找到关于Java的Xpath代码，有的都是把W3School上的文档拷贝过来的，自己也尝试过去用Java去实现遍历，但是发现有的解释不理解，直到看到了这边外国人写的博客，让我瞬间明白了，真的感谢这位哥们。。。
下面是他的原文，我测试过几个列子，都是OK了，大家都懂英文，我就没有必要再翻译过来了，呵呵。
XPath is a language for finding information in an XML file. You can say that XPath is (sort of) SQL for XML files. XPath is used to navigate through elements and attributes in an XML document. You can also use XPath to traverse through an XML file in Java.
XPath comes with powerful expressions that can be used to parse an xml document and retrieve relevant information.
For demo, let us consider an xml file that holds information of employees.
John Watson 30 [email protected] Sherlock Homes 32 [email protected] Jim Moriarty 52 [email protected] Mycroft Holmes 41 [email protected]
I have saved this file at path C:\employees.xml. We will use this xml file in our demo and will try to fetch useful information using XPath. Before we start lets check few facts from above xml file.
- There are 4 employees in our xml file
- Each employee has a unique employee id defined by attribute
emplid - Each employee also has an attribute
typewhich defines whether an employee is admin or user. - Each employee has four child nodes:
firstname,lastname,ageandemail - Age is a number
Let’s get started…
1. Learning Java DOM Parsing API
In order to understand XPath, first we need to understand basics of DOM parsing in Java. Java provides powerful implementation of domparser in form of below API.
1.1 Creating a Java DOM XML Parser
First, we need to create a document builder using DocumentBuilderFactory class. Just follow the code. It’s pretty much self explainatory.
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
//...
DocumentBuilderFactory builderFactory =
DocumentBuilderFactory.newInstance();
DocumentBuilder builder = null;
try {
builder = builderFactory.newDocumentBuilder();
} catch (ParserConfigurationException e) {
e.printStackTrace();
}
1.2 Parsing XML with a Java DOM Parser
Once we have a document builder object. We uses it to parse XML file and create a document object.
import org.w3c.dom.Document;
import java.io.IOException;
import org.xml.sax.SAXException;
//...
try {
Document document = builder.parse(
new FileInputStream("c:\\employees.xml"));
} catch (SAXException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
In above code, we are parsing an XML file from filesystem. Sometimes you might want to parse XML specified as String value instead of reading it from file. Below code comes handy to parse XML specified as String.
String xml = ...; Document xmlDocument = builder.parse(new ByteArrayInputStream(xml.getBytes()));
1.3 Creating an XPath object
Once we have document object. We are ready to use XPath. Just create an xpath object using XPathFactory.
import javax.xml.xpath.XPath; import javax.xml.xpath.XPathFactory; //... XPath xPath = XPathFactory.newInstance().newXPath();
1.4 Using XPath to parse the XML
Use xpath object to complie an XPath expression and evaluate it on document. In below code we read email address of employee having employee id = 3333. Also we have specified APIs to read an XML node and a nodelist.
String expression = "/Employees/Employee[@emplid='3333']/email"; //read a string value String email = xPath.compile(expression).evaluate(xmlDocument); //read an xml node using xpath Node node = (Node) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODE); //read a nodelist using xpath NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
2. Learning XPath Expressions
As mentioned above, XPath uses a path expression to select nodes or list of node from an xml document. Heres a list of useful paths and expression that can be used to select any node/nodelist from an xml document.
nodename |
Selects all nodes with the name “nodename” |
/ |
Selects from the root node |
// |
Selects nodes in the document from the current node that match the selection no matter where they are |
. |
Selects the current node |
.. |
Selects the parent of the current node |
@ |
Selects attributes |
employee |
Selects all nodes with the name “employee” |
employees/employee |
Selects all employee elements that are children of employees |
//employee |
Selects all book elements no matter where they are in the document |
Below list of expressions are called Predicates. The Predicates are defined in square brackets [ ... ]. They are used to find a specific node or a node that contains a specific value.
/employees/employee[1] |
Selects the first employee element that is the child of the employees element. |
/employees/employee[last()] |
Selects the last employee element that is the child of the employees element |
/employees/employee[last()-1] |
Selects the last but one employee element that is the child of the employees element |
//employee[@type='admin'] |
Selects all the employee elements that have an attribute named type with a value of ‘admin’ |
There are other useful expressions that you can use to query the data.
Read this w3school page for more details: http://www.w3schools.com/xpath/xpath_syntax.asp
3. Examples: Query XML document using XPath
Below are few examples of using different expressions of xpath to fetch some information from xml document.
3.1 Read firstname of all employees
Below expression will read firstname of all the employees.
String expression = "/Employees/Employee/firstname";
System.out.println(expression);
NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
Output:
John
Sherlock
Jim
Mycroft
|
3.2 Read a specific employee using employee id
Below expression will read employee information for employee with emplid = 2222. Check how we used API to retrieve node information and then traveresed this node to print xml tag and its value.
String expression = "/Employees/Employee[@emplid='2222']";
System.out.println(expression);
Node node = (Node) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODE);
if(null != node) {
nodeList = node.getChildNodes();
for (int i = 0;null!=nodeList && i < nodeList.getLength(); i++) {
Node nod = nodeList.item(i);
if(nod.getNodeType() == Node.ELEMENT_NODE)
System.out.println(nodeList.item(i).getNodeName() + " : " + nod.getFirstChild().getNodeValue());
}
}
Output:
3.3 Read firstname of all employees who are admin
This is again a predicate example to read firstname of all employee who are admin (defined by type=admin).
String expression = "/Employees/Employee[@type='admin']/firstname";
System.out.println(expression);
NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
Output:
John
Sherlock
|
3.4 Read firstname of all employees who are older than 40 year
See how we used predicate to filter employees who has age > 40.
String expression = "/Employees/Employee[age>40]/firstname";
NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
Output:
Jim
Mycroft
|
3.5 Read firstname of first two employees (defined in xml file)
Within predicates, you can use position() to identify the position of xml element. Here we are filtering first two employees using position().
String expression = "/Employees/Employee[position() <= 2]/firstname";
NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
Output:
John
Sherlock
|
4. Complete Java source code
In order to execute this source, just create a basic Java project in your IDE or just save below code in Main.java and execute. It will need employees.xml file as input. Copy the employee xml defined in start of this tutorial at c:\\employees.xml.
package net.viralpatel.java;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathExpressionException;
import javax.xml.xpath.XPathFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.SAXException;
public class Main {
public static void main(String[] args) {
try {
FileInputStream file = new FileInputStream(new File("c:/employees.xml"));
DocumentBuilderFactory builderFactory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder = builderFactory.newDocumentBuilder();
Document xmlDocument = builder.parse(file);
XPath xPath = XPathFactory.newInstance().newXPath();
System.out.println("*************************");
String expression = "/Employees/Employee[@emplid='3333']/email";
System.out.println(expression);
String email = xPath.compile(expression).evaluate(xmlDocument);
System.out.println(email);
System.out.println("*************************");
expression = "/Employees/Employee/firstname";
System.out.println(expression);
NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
System.out.println("*************************");
expression = "/Employees/Employee[@type='admin']/firstname";
System.out.println(expression);
nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
System.out.println("*************************");
expression = "/Employees/Employee[@emplid='2222']";
System.out.println(expression);
Node node = (Node) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODE);
if(null != node) {
nodeList = node.getChildNodes();
for (int i = 0;null!=nodeList && i < nodeList.getLength(); i++) {
Node nod = nodeList.item(i);
if(nod.getNodeType() == Node.ELEMENT_NODE)
System.out.println(nodeList.item(i).getNodeName() + " : " + nod.getFirstChild().getNodeValue());
}
}
System.out.println("*************************");
expression = "/Employees/Employee[age>40]/firstname";
nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
System.out.println(expression);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
System.out.println("*************************");
expression = "/Employees/Employee[1]/firstname";
System.out.println(expression);
nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
System.out.println("*************************");
expression = "/Employees/Employee[position() <= 2]/firstname";
System.out.println(expression);
nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
System.out.println("*************************");
expression = "/Employees/Employee[last()]/firstname";
System.out.println(expression);
nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
System.out.println("*************************");
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (SAXException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (ParserConfigurationException e) {
e.printStackTrace();
} catch (XPathExpressionException e) {
e.printStackTrace();
}
}
}
That’s all folks :)
针对上述例子，我又想到了一种情况，发现他里面没有涉及到，于是我就尝试啊，也看了Xpath的源代码，发现还是要通过挨个遍历的方式去找，一个笨办法给大家演示下，谁如有好的方法可以给我说下，大家共享下呗。。。
我的测试代码：
package com.fit.test01;
import java.io.IOException;
import java.io.InputStream;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathExpressionException;
import javax.xml.xpath.XPathFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.SAXException;
public class XPathEmployee {
public static void main(String[] args) {
DocumentBuilderFactory builderFactory = DocumentBuilderFactory
.newInstance();
DocumentBuilder docbuilder;
InputStream is = null;
try {
is = XPathEmployee.class.getClassLoader().getResourceAsStream(
"employees.xml");
// 一种是获取当前文件的路径，一种是获取当前文件的流，两种方式都可以,并且文件应该在当前工程的src目录下
// String strFilePath =
// XPathEmployee.class.getClassLoader().getResource("employees.xml").toString();
docbuilder = builderFactory.newDocumentBuilder();
Document xmlDocument = docbuilder.parse(is);
XPath xPath = XPathFactory.newInstance().newXPath();
Node node = null;
NodeList nodeList = null;
String expression = "/Employees/Employee[@type='admin']";
nodeList = (NodeList) xPath.compile(expression).evaluate(
xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
node = nodeList.item(i);
if (node.getNodeType() == Node.ELEMENT_NODE) {
System.out.println(node.getNodeName() + " : "
+ node.getFirstChild().getNodeValue());
//这个时候才到employee层，所以需要向下再延伸。。。
if (node.hasChildNodes()) {
System.out.println("----------------");
NodeList nodeList1 = node.getChildNodes();
for (int j = 0; j < nodeList1.getLength(); j++) {
Node node1 = nodeList1.item(j);
if (node1.getNodeType() == Node.ELEMENT_NODE) {
System.out.println(node1.getNodeName() + " : "
+ node1.getFirstChild().getNodeValue());
}
}
}
}
}
} catch (SAXException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (ParserConfigurationException e1) {
e1.printStackTrace();
} catch (XPathExpressionException e) {
e.printStackTrace();
} finally {
if (is != null) {
try {
is.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
}
运行结果如下（主要想测试，如果有多个Node的情况，他的例子中都是精确到了属性，结果都是唯一的一个，那如果有多个呢？见上述方法）：
Employee : ---------------- firstname : John lastname : Watson age : 30 email : [email protected] Employee : ---------------- firstname : Sherlock lastname : Homes age : 32 email : [email protected]